In known air classification systems for receiving mixed materials and separating them into respective groups of light and heavy materials there is employed a rotary drum air classifier. The drum is disposed at an angle to the horizontal and air is forced at high velocity through it, in at the lower end and out the higher end. Mixed materials are supplied by a belt conveyor to a midpoint along the length of the drum interior where they fall from the conveyor onto the drum wall. As the drum rotates about its axis, longitudinally extending lifters on the drum wall carry the materials upwardly to a point where they again fall by gravity onto the drum wall; this time, however, because of the drum's angle they fall at a point nearer the lower end of the drum.
During such dropping of the mixed materials from the end of the conveyor and from the lifters the high velocity air stream will entrain within it light materials such as paper and the like and will carry such light materials out the upper end of the drum to be collected for further processing or disposal.
The heavy materials, however, will be repetitively raised and dropped inside the drum as it rotates until eventually they will fall out the lower end of the drum for subsequent further processing or disposal.
However, it has been found that unenclosed or canopied belt conveyors are somewhat unsuitable for feeding mixed materials into the drum from a supply hopper since the conveyors usually are located in such a position that they will intercept some of the heavy materials as they fall after being raised by the lifters or vanes during the separation procedure. Furthermore, in municipal waste there are often included substantial amounts of wire. It has been found that when unenclosed belt conveyors are used it is very easy for pieces of wire to become entangled in the mechanism. It will be apparent that this can seriously interfere with the operation of the device.
With prior art rotary drum air classifiers it has been believed necessary to deposit the mixed materials within the drum at a point which is centrally disposed along the longitudinal dimension or length of the drum. However, it has been found that this requirement is not necessary and in fact can unnecessarily extend the operative time for efficient classification.
An additional objection to prior art rotary drum air classification systems is the fact that the feed conveyor is of fixed length and continuously deposits mixed materials at a single location in the drum regardless of the conveyor's varying loads, thus sometimes causing clogging of the drum and requiring that steps be taken to shut down the conveyor and unclogging the drum before additional materials can be processed.